1. Field of the Invention
The present invention relates generally to optical devices, and more particularly to a method and apparatus for monitoring the performance of each of a plurality of channels propagated via an optical medium.
2. Description of the Prior Art
The explosive growth of telecommunication and computer communications, especially in the area of the Internet, has created a dramatic in increase in the volume of worldwide data traffic which has placed an increasing demand for communication networks providing increased bandwidth. To meet this demand, fiber optic (light wave) communication systems have been developed in order to harness the enormous usable bandwidth (tens of tera-Hertz) of a single optical fiber transmission link. Because it is not possible to exploit all of the bandwidth of an optical fiber using a single high capacity channel, wavelength division-multiplexing (WDM) fiber optic systems have been developed to provide transmission of multi-carrier signals over a single optical fiber thereby channelizing the bandwidth of the fiber. In accordance with WDM technology, a plurality of superimposed concurrent signals are transmitted on a single fiber, each signal having a different wavelength. WDM technology takes advantage of the relative ease of signal manipulation in the wavelength, or optical frequency, domain, as opposed to the time domain. In WDM networks, optical transmitters and receivers are tuned to transmit and receive on a specific wavelength, and many signals operating on distinct wavelengths share a single fiber.
Wavelength multiplexing devices are commonly used in fiber optic communication system to generate a single multi-carrier main communication signal stream, in response to a plurality of concurrent signals having different wavelengths received from associated sources or channels, for transmission via a single fiber. At the receiving end, wavelength demultiplexing devices are commonly used to separate the composite wavelength signal into the several original signals having different wavelengths.
The transmission capacity of an optical network is proportional to the number of channels carried by the main communication signal stream in the optical network. Dense wavelength division multiplexing (DWDM) systems provide many channels with narrowly spaced wavelength separations, such as 50, or 100 GHz channel spacing which corresponds to a wavelength separation of 0.4 nm and 0.8 nm, respectively. The number of channels deployed in WDM optical networks is continually increasing. Currently, it is common for WDM optical networks to deploy 16, 32, and 40 channels. DWDM systems providing very large number of channels (e.g., 80 and 160 channels) are likely to be deployed in the foreseeable future.
In optical networks having a large number of channels, the stability of the channels (both in terms of the amplitude and wavelengths) is critical. The stability of channels in optical networks is largely dependent on the operational characteristics of optical transmitters used in the network. It is well known that as lasers age and as the operational temperature of the lasers change, the wavelengths and amplitude of the light emitted by the lasers tends to vary. As the number of channels deployed in an WDM optical network increases, wavelength drifts are more likely to result in interference between channels because the channel spacing is narrower. Therefore, as the number of channels deployed in an WDM optical network increases, wavelength drifts and amplitude variations are increasingly more likely to cause data error or transmission failures.
It is important for engineers to be able to monitor the performance of the communications channels in an optical network. Conventional optical network performance monitoring devices typically include a detection element responsive to the main communication signal stream including all of the channels carried by the network. In these conventional devices, the element is responsive to the combined amplitudes of all of the channels carried by the main signal stream, and operative to generate a data signal indicative of the power level provided collectively by all of the channels of the main communications signal stream. This data signal is used to determine the overall health, or performance level, of all of the channels of the network.
As the number of channels increases and as the channel spacing is reduced in optical networks, conventional optical performance monitoring devices become less effective and less useful because such detectors can only be used detect the total power provided collectively by all wavelengths of the main communication signal stream. Conventional optical performance monitoring devices do not provide reliable information indicating the performance of individual channels of the main communication signal stream of as network. If the power level of one of the channel decreases, while the power level of another one of the channel increases, the total power level measured by the conventional device for the main communication signal stream may remain constant, thereby providing an inaccurate indication of the performance of the network.
What is needed is an optical performance monitoring system that provides for monitoring the performance of each of a plurality of channels of a main signal stream propagated via an optical network.
It is therefore an object of the present invention to provide an optical performance monitoring system that provides for monitoring the performance of each of a plurality of channels of a main signal stream propagated via an optical network.
Another object of the present invention to provide an optical performance monitoring system that provides wavelength and amplitude information associated with each of the channels of an optical network.
A further object of the present invention to provide a compact and stable optical performance monitoring device that yields precise information data indicating the wavelengths and amplitudes associated with each of the channels of an optical network.
Another important object of the present invention is to provide DWDM device that is easy to manufacture in large quantities using components that are easy to make and assemble.
Briefly, a presently preferred embodiment of the present invention provides a system for monitoring the optical performance of a fiber optic network including an optical fiber carrying a main communication signal stream having a plurality of different channels each having an associated channel wavelength. The system includes: a fiber tapping means communicatively coupled with the main optical fiber for extracting a tapped communication signal from the main signal stream, the tapped communication signal including the plurality of channels each having an associated communication wavelength; means for generating a calibration signal including a plurality of calibration wavelengths; means for combining the tapped communication signal and the calibration signal to generate a combined signal including the communication wavelengths and the calibration wavelengths; and a multi-channel monitoring device.
The multi-channel monitoring device includes: an input optical fiber for receiving the combined signal, and having an input fiber end for radiating an input light beam including the communication wavelengths and the calibration wavelengths; a detector unit having an array of pixel detector elements each being responsive to light beams incident thereon, and operative to generate an associated data signal; collimating and focusing means for collimating and focusing beams propagating between the fiber end and the detector unit; and a transmissive grating assembly for diffracting and spatially separating beams propagating between the input fiber end and the detector unit. The grating assembly, collimating and focusing means, and pixel detector elements are configured so that the diffracted beams are spatially separated and focused onto associated ones of the detector elements according to associated ones of the communication wavelengths and the calibration wavelengths. In an embodiment, the grating assembly includes a volume phase grating (VPG) fabricated from high efficiency DCG material. The grating is disposed at a predetermined angular orientation with respect light beams incident thereon in order to optimize diffraction efficiency, and to optimize a first order of diffraction for a predetermined wavelength range.
The system further includes: a data processing unit for receiving the data signals from the detector unit, and being operative to provide information indicative of the intensities and wavelengths associated with each of the channels of the main communication signal. A display unit may be used to display graphical indicia, including spectral diagrams, indicative of the performance of each of the communication channels. The data signal processing unit is operative to perform a calibration process for calibrating positional relationships between the spatially diffracted beams associated with the communication wavelengths and the associated pixel detector elements, the calibration process using the calibration wavelengths as standards to calibrate the positional relationships.
Also in an embodiment, the means for generating a calibration signal includes: a light source providing a source light beam; a plurality of bandpass filters each being communicatively coupled to receive the source light beam from the source, and being operative to filter the source light beam, and to pass an associated one of the plurality of calibration wavelengths; and means for combining the calibration wavelengths passed by the filters to provide the calibration signal. In one embodiment, each of the bandpass filters includes a fiber Bragg grating.
Important advantages of the optical performance monitoring device of the present invention is that it is compact and stable, and yields precise performance information indicative of the wavelengths and amplitudes associated with each of the channels of an optical network
The forgoing and other objects, features, and advantages of the present invention will be apparent from the following detailed description of the preferred embodiment which makes reference to the several figures of the drawing.